Pyrolysis is the process of thermally decomposing an organic material in either a reduced oxygen environment or an oxygen-free environment. Pyrolyzing wood and other forms of mixed biomass ultimately produces a carbonaceous compound called biochar and a mixture of gases including hydrocarbons, oxygenates, water, hydrogen, carbon monoxide and carbon dioxide. If the vapors are condensed, the liquid produced is sometimes referred to as bio-oil and the volatile gases are sometimes referred to as synthesis gas (syngas).
Biomass, such as cellulose, hemicelluloses, lignin, starches, and lipids, proceeds through multiple steps of decomposition when subject to the pyrolysis process. In general, when conducted at high temperatures (e.g., 800° C.) for prolonged periods of time, pyrolysis ultimately yields syngas. As the temperature and exposure time interval is reduced, an increasing amount of biochar residue remains. At still lower temperatures and time intervals, increasingly complex hydrocarbons and oxygenated hydrocarbons are present in the gas stream from the pyrolyzed biomass.
Wood vinegar, also known as pyroligneous acid, may be extracted as a by-product in the manufacture of biochar from cellulose containing biomass. It typically is produced as the first cut, after water, in the pyrolysis of cellulosic biomass and is usually generated at temperatures below 320° C. Wood vinegar typically is about 80-90% water and contains acetic acid, methanol and acetone in addition to a number of other compounds, including carboxylic acids and other organic acids. The pyrolysis stream containing wood vinegar is usually discarded.
In other pyrolysis systems, the biomass is pretreated prior to the pyrolysis and leads to a waste stream(s) with organic acids. This can be a separate pyrolysis step known as torrefaction, where the biomass is heated to moderate temperatures, generally between 200° C. and 320° C., and releases water and organic acids from the biomass and leaves the process as a waste stream. Another pretreatment step often utilized is pre-washing the biomass with acid and/or hot water, which also produces a waste stream containing water and acid.
Additionally, the vapors produced from pyrolysis contain organic acids, including but not limited to acetic acid, formic acid, propionic acid, glycolic acid, and other carboxylic acids. Additionally these vapors may also contain inorganic acids, including but not limited to hydrochloric acid. Dependent on downstream processing steps, a waste stream with organic acids, like acetic acid, and potentially inorganic acids, like hydrochloric acid, and water may be created downstream of the pyrolysis reaction from this pyrolysis vapor stream. This waste stream is typically just discarded as well.
It would be beneficial to utilize any of these discarded acid containing by-products of pyrolysis.